In a fuel cell using polymer electrolyte, electric power and heat are simultaneously generated by electrochemically reacting a fuel gas containing hydrogen with a fuel gas, such as air, containing oxygen. This fuel cell is basically constituted by a pair of electrodes, i.e. anode and cathode, formed on opposite surfaces of a polymer electrolyte membrane, which selectively transports hydrogen ions. The above electrodes are each comprising a catalyst layer having, as a main constituent, a carbon powder carrying a platinum group metal catalyst, and a diffusion layer having both gas permeability and electronic conductivity.
Furthermore, in order to prevent the fuel gas and the oxidant gas, to be supplied to the electrodes, from leaking to outside, and to prevent the two kinds of gases from intermingling with each other, a gas sealing material and a gasket are arranged, at a periphery of the electrodes, to sandwich the polymer electrolyte membrane. These sealing material and gasket are preliminarily assembled integrally with the electrodes and the polymer electrolyte membrane. This is called MEA (electrolyte membrane-electrode assembly). Outside the MEA, electrically conductive separator plates for electrically connecting neighboring MEAs in series are arranged. A gas flow channel for supplying a reactive gas to the electrode surface and for carrying away a produced gas and an excessive gas is formed at a portion of the separator, which portion is to contact with an MEA. The gas flow channel can be provided separately from the separator plate, but such manner is generally used as to provide a groove, as a gas flow channel, at the surface of each separator plate.
In order to supply a fuel gas and an oxidant gas to such grooves, it becomes necessary to prepare a piping jig which divides pipes, for respectively supplying the fuel gas and the oxidant gas, into a number of branches corresponding to the number of used separator plates, and which directly and fittedly connects the respective branches to the grooves of the separator plates. This jig is called manifold. The above type of manifold, which enables direct and fitted connection from the supply pipes for the fuel gas and the oxidant gas, is called external manifold. There is another type of manifold, called internal manifold, which has a simpler structure. In the internal manifold, a separator plate having a gas flow channel formed thereon is provided with through-holes, and the inlet and outlet of the gas flow channel are connected to the through-holes so as to enable direct supply of the fuel gas and the oxidant gas.
Since a fuel cell generates heat during its operation, it needs to be cooled by e.g. a cooling water in order to maintain the cell at a good temperature state. Usually, a cooling member to flow a cooling water is provided for every 1 to 3 cells. In one type of such, the cooling member is inserted between a separator plate and a separator plate. In another type, a cooling water flow channel is provided, as a cooling member, at a rear surface of a separator plate. The latter type is more often used. In a general cell stack structure, these MEAs, separator plates and cooling members are alternately stacked so as to form a stack of 10 to 200 cells, and such stacked body is sandwiched by end plates via a current collecting plate and an insulating plate, the both ends being fixed by tightening bolts.
In such a polymer electrolyte fuel cell, the separator plates need to have a high electric conductivity, a high gas tightness against the fuel gas and the oxidant gas, and further a high corrosion resistance to the reaction upon oxidation/reduction of hydrogen/oxygen. For these reasons, a conventional separator plate was usually constituted by a carbon material such as glassy carbon and expanded graphite, wherein a gas flow channel was made by cutting the surface of the plate, or by molding using a mold in the case of expanded graphite.
According to the conventional carbon plate cutting method, it was difficult to lower the cost of the carbon plate material and the cost of cutting it. Likewise, in the case of expanded graphite, the material cost is high. These are considered as obstacles for commercialization.
Recently, the use of a metal plate such as stainless steel is attempted in place of the carbon material having been conventionally used.
However, in the above case of using the metal plate, corrosion and dissolution of the metal plates occur after a long period use, because the metal plate is exposed to an oxidizing atmosphere of about pH 2 to 3 at a high temperature. When the metal plate gets corroded, its electric resistance at the corroded portion increases, and the output of the cell decreases. Further, when the metal plate gets dissolved, the dissolved metal ions are diffused in the polymer electrolyte, and are trapped at ion exchange sites of the polymer electrolyte, whereby consequently the polymer electrolyte itself gets lowered in its ionic conductivity. For these causes, it has been a problem that when a metal plate, as is, is used for the separator plate, and the cell is operated for a long period, its power generation efficiency gradually gets lowered.
In order to avoid such deterioration, it has been a usual way to subject the surface of the metal plate to gold plating having some thickness. Furthermore, such separator as being made of an electrically conductive resin made by mixing a metal power in e.g. an epoxy resin has been studied. (See Japanese Laid-Open Patent Publication Hei 6-333580.)
As described above, in the case of a method of making a separator by cutting a glassy carbon plate, the material cost per se of the glassy carbon plate is high, and further it is difficult to lower the cost for cutting it. Expanded graphite having been press-machined has a problem of mechanical strength of the material. When it is used as a power source of an electric vehicle, cracks may be produced therein due to vibrations and shocks during driving. The separator made of a metal plate having gold plating has a problem of the gold plating cost. In the case of a separator made of an electrically conductive resin, its electric conductivity is lower than that of glassy carbon or a metal plate, and further the surface of the resin is hard. Therefore, in order to lower its electric resistance at a contact portion thereof with an electrode, tightening by a strong pressure is necessary, whereby the resultant cell structure becomes complicated.